1. Field of the Disclosure
This invention pertains to a method for printing a material onto a substrate, and in particular, to a method of printing using a relief printing form that transfers the material from a plurality of raised surfaces upon contact to the substrate.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to cardboard boxes and to continuous web of plastic films. Flexographic printing plates are used in relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Flexographic printing plates are characterized by their ability to crosslink or cure upon exposure to actinic radiation. The plate is imagewise exposed with actinic radiation through an image-bearing art-work or a template, such as a photographic negative or transparency (e.g., silver halide films) for so called analog workflow, or through an in-situ mask having radiation opaque areas that had been previously formed above the photopolymerizable layer for so called digital workflow. The actinic radiation exposure is typically conducted with ultraviolet (UV) radiation. The actinic radiation enters the photosensitive element through the clear areas and is blocked from entering the black or opaque areas of the transparency or in-situ mask. The areas of the photopolymerizable layer that were exposed to the actinic radiation crosslink and harden and/or become insoluble to solvents used during development. The unexposed areas of the photopolymerizable layer that were under the opaque regions of the transparency or the in-situ mask during exposure do not hardened and/or remain soluble. The unexposed areas are removed by treating with a washout solution or heat leaving a relief image suitable for printing. If treated with washout solution, the plate is dried. After all desired processing steps, the plate is then mounted on a cylinder and used for printing.
A flexographic-like printing process is used today to coat polyimide materials onto surfaces in the construction of a liquid crystal display device (LCD). The polyimide material is used as an orientation layer in a liquid display device. A flexographic relief printing form is fundamentally used as a coating plate with typically a 400 line per inch (LPI) and a 40% image in an area/s of the plate that transfers the polyimide material to form a coating on the substrate. A coating plate is a printing form, such as a plate or cylinder, having a relief pattern that is intentionally designed to provide a uniform coating (i.e. layer) of a material on selected regions of the substrate. The relief printing form includes a relief structure having one or more areas, i.e., coating areas, composed of a plurality of fine, closely-spaced, raised surface elements that transfer the coating material to the substrate; and having one or more recessed areas that do not pickup the coating material and that form non-coated areas on the substrate. Upon transfer, the plurality of fine, closely-spaced raised surface elements create a uniform layer of the coating material on the substrate, i.e., a continuous layer of the coating material on the contacted region of the substrate. The relief printing form transfers the coating material from the one or more coating areas to the substrate to form a pattern of coated regions and non-coated regions, where the coated regions each have the uniform layer of the coating material on the substrate. The relief printing form with the plurality of fine, closely-spaced raised surface elements is necessary to meet the predefined specifications and to create the pattern of uniform layer of the coating material in the selected regions on the substrate. Although it is possible to create a relief printing form having one or more coating areas composed of a singular, raised surface element instead of the plurality of fine closely-spaced raised surface elements, transfer of the coating material by the singular raised surface element does not create a sufficiently uniform layer of the coating material in the coated regions on the substrate.
Currently relief printing plates or printing forms for coating polyimide materials are fabricated by either of two methods from photosensitive precursors. One method to fabricate the relief printing form utilizes a sheet plate material, such as, for example, CYREL® NOW (from DuPont, Wilmington, Del.) photopolymerizable printing precursor, and a second method utilizes liquid plate manufacturing workflow, such as, for example, Asahi APR K-11. The sheet plate fabrication involves conventional solid plate-making steps with analog workflow used to imagewise expose the plate through the silver halide photographic negative as described above. The liquid plate fabrication involves conventional multi-step liquid plate-making process that includes making an intermediate by etching a chrome coating of an image on a glass substrate.
In other printing applications, after printed matter is formed either before or after the ink has dried, it is conventional to apply a coating over selected areas of the printed substrate. This process is generally termed spot or pattern coating. Spot coating constitutes applying a coating in small areas surrounded by a lack of coating in large surrounding areas. Alternatively, a coating termed “pattern coating” may be applied in large areas with the absence of coating in small selected areas. The coating may be applied for various reasons including to protect the printed material; to prevent the printed material from sticking to other sheets when placed in a pile of sheets, when the ink is still wet or tacky; and, to improve scuff resistance. The coating may also be applied to selected areas for aesthetic reasons. For example, it is sometimes desired to provide gloss to certain areas of the sheet in order to provide highlighting while leaving other areas dull. Additionally, when certain areas of printing folding cartons must be coated with an adhesive, pattern coating is used to avoid the adhesive coated areas. Hereto, the relief printing form should include a relief structure having one or more coating areas that is composed of a plurality of fine, closely-spaced, raised surface elements to transfer the coating material and form a suitable coating layer on the substrate.
Both the solid and liquid plate workflows involve making an intermediate, i.e., the photographic negative and the chrome patterned glass. Analog workflow requires the preparation of the phototool, which is a complicated, costly and time-consuming process requiring separate processing equipment and chemical development solutions. In addition, the phototool may change slightly in dimension due to changes in temperature and humidity. The same phototool, when used at different times or in different environments, may give different results. Use of a phototool also requires special care and handling when fabricating flexographic printing forms to ensure intimate contact is maintained between the phototool and plate. In particular, care is required in the placement of both the phototool and the plate in the exposure apparatus along with special materials to minimize air entrapment during creation of a vacuum to ensure intimate contact. Additionally care must be taken to ensure all surfaces of the photopolymer plate and phototool are clean and free of dust and dirt. Presence of such foreign matter can cause lack of intimate contact between the phototool and plate as well as image artifacts.
An alternative to analog workflow is termed digital workflow, which does not require the preparation of a separate phototool. Photosensitive elements suitable for use as the precursor and processes capable of forming an in-situ mask in digital workflow are described in U.S. Pat. No. 5,262,275; U.S. Pat. No. 5,719,009; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,238,837; U.S. Pat. No. 6,558,876; U.S. Pat. No. 6,929,898; U.S. Pat. No. 6,673,509; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,037,102; and U.S. Pat. No. 6,284,431. The precursor or an assemblage with the precursor includes a layer sensitive to infrared radiation and opaque to actinic radiation. The infrared-sensitive layer is imagewise exposed with laser radiation whereby the infrared-sensitive material is removed from, or transferred onto/from a superposed film of the assemblage, to form the in-situ mask having radiation opaque areas and clear areas adjacent the photopolymerizable layer. The precursor is exposed through the in-situ mask to actinic radiation in the presence of atmospheric oxygen (since no vacuum is needed). Furthermore, due in part to the presence of atmospheric oxygen during imagewise exposure the flexographic printing form has a relief structure that is different from the relief structure formed in analog workflow (based upon the same size mask openings in both workflows). Digital workflow creates a raised element (i.e., dot) in the relief structure having a surface area of its uppermost surface (i.e., printing surface) that is significantly less than the opening in the in-situ mask corresponding to the relief structure. Digital workflow results in the relief image having a different structure for dots (i.e., raised surface elements) that is typically smaller, with a rounded top, and a curved sidewall profile, often referred to as dot sharpening effect. Dots produced by analog workflow are typically conical and have a flat-top. The relief structure formed by digital workflow results in positive printing properties such as, finer printed highlight dots fading into white, increased range of printable tones, and sharp linework. As such, the digital workflow because of its ease of use and desirable print performance has gained wide acceptance as a desired method by which to produce the flexographic printing form. But not all end-use applications view this dot-sharpening effect as beneficial.
It is known by those skilled in the art that the presence of oxygen (O2) during exposure in free-radical photopolymerization processes will induce a side reaction in which the free radical molecules react with the oxygen, while the primary reaction between reactive monomer molecules occurs. This side reaction is known as inhibition (i.e., oxygen inhibition) as it slows down the polymerization or formation of crosslinked molecules. Many prior disclosures acknowledge that it is desirable for photopolymerization exposure to actinic radiation to occur in air (as is the case for digital workflow), under vacuum (as is the case for analog workflow), or in an inert environment. Oftentimes, nitrogen is mentioned as a suitable inert gas for the inert environment. The implication is that the nitrogen environment is one that contains substantially less than atmospheric oxygen to the extent that all oxygen is removed, or something less than about 10 ppm of oxygen. Nitrogen with oxygen impurity concentration level less than 10 ppm is readily commercially available.
There is a desire to eliminate the costs and the time consuming and problematic process steps associated with the preparation of the photographic negative intermediate, and transition from analog workflow to digital workflow in the fabrication of solid plates that are capable of transferring a coating material to form a layer of the coating material on a substrate. However, the dot-sharpening effect associated with conventional digital exposure in the presence of atmospheric oxygen becomes a disadvantage for relief printing forms, such as the coatings plate, that need to have a relief structure that is generated by high line resolution imaging. Because imagewise exposure in digital workflow is conducted in the presence of atmospheric oxygen which inhibits polymerization, the dot structure of the raised surface element has a printable surface area that is considerably reduced (compared to that produced by analog workflow, as well as compared to the corresponding opening in the digital mask image). The magnitude of the reduction in printable dot surface area made by the conventional digital workflow is such that the plurality of raised printing elements in the coating area of the printing form are not sufficiently close enough that a uniform layer of the coating material can be formed on the substrate. The number of raised surface elements and the proximity of the raised elements to each other in a given coating area of the relief printing form must be maintained at the high line screen resolution, i.e., greater than 250 lines per inch, in order for the printing form to provide the desired uniformity of the layer of coating material on the substrate. As was explained above, it is necessary for the printing form to have coating areas with a high resolution relief pattern, i.e., plurality of fine, closely-space raised surface elements, in order to transfer and form a uniform layer of the coating material on the substrate. Further, the magnitude of the reduction in printable dot surface area made by the digital workflow cannot be compensated by creating larger openings in the in-situ mask because each opening would need to be larger than the dimension of a pixel associated with the desired line/dot resolution, and would overlap into the adjacent pixel. Consequently, it is not possible by conventional digital workflow to fabricate a relief printing form that has a plurality of fine and closely-spaced, raised surface elements (i.e., dots) in a coating area sufficient to transfer a coating material and form a uniform continuous layer of the coating material on the substrate.
In other end-use applications, it is desirable to create a relief surface for a flexographic printing form that is suitable for printing and accurately reproduces a pattern of high-line screen resolution images and/or text on desired document or other item. Security printing is one such end-use application which is placing increasing demands for reproduction of high-resolution images. Security printing deals with the printing of items, such as banknotes, passports, tamper-evident labels, stock certificates, postage stamps, and identity cards, in order to prevent forgery, tampering, or counterfeiting of the document or object. One example of high-resolution images used in security printing is microprinting. Microprinting involves the use of extremely small text, and is most often used on currency and bank checks. The text is generally small enough to be indiscernible to the naked eye. In such high-line screen resolution printing applications, the raised surface elements in a given coating area of the relief printing form should be formed at high line screen resolutions, i.e., equal to or greater than 250 lines per inch, so that each of the raised surface elements are sufficiently fine and well-structured to be capable of accurately reproducing the desired fine image pattern on the substrate, document, or other item. Hereto, it is desirable to fabricate the printing form using digital workflow due to its ease and simplicity, while providing a relief structure in the printing form similar to or the same as the relief structure created by analog workflow.
Thus, the need to use digital workflow in the fabrication of relief printing forms conflicts with the need for the printing form to have high line resolution raised surface elements similar to the size and proximity of the raised surface elements produced by analog workflow. So a need arises for a method of fabricating a relief printing form from a photosensitive precursor element that utilizes digital workflow to eliminate the costly and problematic creation of a separate intermediate. The relief printing form needs to have a relief structure composed of recessed areas and raised areas for forming a pattern of printing regions on a substrate, wherein each the raised area is composed of a plurality of fine, closely-spaced, raised surface elements capable of transferring an imaging material, such as an ink, varnish, orientation material, or other coating material, to create the desired image in the printing regions on the substrate.